How Much Does Plug-in Solar Save? Real UK Numbers by System Size

Every homeowner considering plug-in solar asks the same question: will it actually save me money? The answer is yes—but the amount depends entirely on your location, system size, and how much of your own electricity you use.

This article lays out real, ballpark savings figures for 400W, 600W, and 800W systems across the UK, so you can see what different sizes actually generate and save. No calculator needed yet—just the data.

The Real Numbers: 400W, 600W, 800W Compared

These figures are based on PVGIS data for four representative UK locations, assuming:

• South-facing, tilted at 25° (standard roof mount)
• No shading
• 19% panel efficiency (typical modern modules)
• Standard electricity tariff at 30p/kWh
• No battery storage (all surplus exported to grid)

London (South-East England)

London is the sunniest major UK city, with ~1,050 kWh/m²/year of solar irradiance. A 400W system barely pays for itself if installed in a flat with flat roof; 800W makes solid economic sense over 15–20 years.

Bristol (South-West England)

Bristol's solar irradiance is ~980 kWh/m²/year—slightly lower than London, but still excellent. The West Country's mix of clear spring mornings and cloudy afternoons still yields solid generation.

Manchester (North-West England)

Manchester and surrounding areas get ~920 kWh/m²/year of solar irradiance. Lower than the South, but still profitable. The slightly longer payback reflects higher cloud cover.

Edinburgh (Scotland)

Scotland's solar irradiance is ~800 kWh/m²/year. The longer days in summer compensate for lower altitude sun in winter, but cloud cover is the limiting factor. Still, even Edinburgh's payback at under 4 years beats most investments.

Why Payback Is Similar Across System Sizes

Notice that payback period stays roughly constant (3.1–3.9 years) regardless of system size? This is because the cost-per-watt scales with the system size. A 400W system costs ~£400–£500; an 800W costs ~£800–£1,000. Generation scales linearly, so the ratio stays the same.

What changes is absolute annual savings: 800W saves you twice as much as 400W, but costs twice as much upfront. The payback timeline is identical.

This matters for your decision: you don't choose system size to improve payback; you choose it based on your available space and budget. All three sizes are equally "worth it" from a financial perspective over a 15–20 year lifespan.

Best overall

EcoFlow [PowerStream](/blog/ecoflow-stream-review-uk) 800W Kit

~£799 estimated

• 800W dual-panel balcony or garden kit
• 600Wh plug-in battery included
• App with live monitoring & smart scheduling
• Balcony, garden & flat-roof mount options

View on EcoFlow →

BSI-compliant kits expected from July 2026. Prices are estimates.

The Impact of Tariff: Why 30p/kWh Is a Baseline

The savings figures above assume a standard variable-rate tariff at ~30p/kWh. But here's where plug-in solar gets interesting: your actual tariff dramatically changes the maths.

Standard Fixed or Variable Rate (30p/kWh)

This is your baseline. Most standard tariffs pay 0p for exported solar (net metering is not standard in the UK for domestic customers). You save money only on what you self-consume (use immediately).

Example: 800W system in London saves ~£256/year.

Economy 7 (30p day, 10p night)

If you have Economy 7, most of your solar generation happens during the 30p period (daytime), so savings are similar to standard rates. You gain no advantage for exporting.

Example: 800W system in London saves ~£255/year (nearly identical, as most generation is daytime anyway).

Time-of-Use Tariff (Octopus Go, Flux, etc.)

Many newer ToU tariffs charge 15–20p during daytime "peak" hours and 5–8p off-peak.

If your solar is generating during the peak-rate hours (roughly 8am–8pm), you save money at the higher rate. This is even more valuable than exporting to the grid.

Example: 800W system in London on Octopus Flux (15p peak) saves ~£255/year from self-consumption alone, which is actually similar to the 30p standard—but the effective benefit is higher because you're saving peak-rate power that would have cost you more.

However, if you then export surplus solar during off-peak hours, some tariffs pay 15p per exported unit (Octopus Flux pays export rewards). That stacks on top.

Export Tariff (SEG – Smart Export Guarantee)

The UK's Smart Export Guarantee (SEG) allows small generators to sell surplus power. Rates are currently 15–20p per unit, depending on supplier. If your system exports 50% of its generation, you earn ~£40–£80/year extra.

Example: 800W system in London with SEG export at 15p might save £256 self-consumption + ~£60 export = ~£316/year.

This is why our tariff guide matters so much. The same system in the same location can save you 20–50% more if you switch to a tariff that rewards solar generation.

Monthly Variation: Why Summer Savings Are 5× Winter

Solar generation isn't constant. Summer months generate far more than winter, which catches many first-time solar users by surprise.

Here's a typical monthly breakdown for an 800W south-facing system in London:

Notice June–August generate ~3× more than December–February. This is the seasonal swing: longer days, higher sun angle, and typically clearer skies in summer.

If you're thinking "I'll save £256 a year," that's accurate on average—but don't expect £21/month every month. June might save you £34; December might save you £10.

This seasonal pattern is why battery storage becomes relevant: if you want to "flatten" your savings across the year (capturing more evening generation), a battery helps. But it's expensive for the benefit; most users simply accept the seasonal variation.

The £1/W Rule of Thumb

A practical shortcut: assume payback in 3–4 years if the system costs ~£1 per watt.

• £400 system = 400W → payback in 3–4 years
• £800 system = 800W → payback in 3–4 years
• £1,200 system = 1,200W → payback in 3–4 years

This rule breaks down if you get a very cheap system (£0.80/W) or a very expensive one (£1.50/W), but it's a useful benchmark while shopping.

July 2026 will bring BSI-compliant kits to the market. Current estimates suggest they'll be priced in the £500–£1,200 range depending on brand and size. Once they're live, check the buying guide for current prices.

Special Cases: When Savings Are Higher or Lower

Savings Are Higher If:

• You have a Time-of-Use tariff – 15–20p peak rates mean each unit of self-consumed solar is worth more.
• You have high daytime usage – work from home, electric heating, or frequent laundry means more self-consumption and fewer exports at 0p.
• You're in the South – London, Bristol, or the Midlands get 15–20% more generation than the North or Scotland.
• You have south or south-west aspect – shaving 5–10% off payback vs. east-west.
• You have a battery – though expensive, it captures more of your generation for evening use. Read the battery vs no-battery guide.

Savings Are Lower If:

• You're on Economy 7 – most savings come during the 30p daytime period anyway, so you're not using off-peak solar well.
• You work away from home – little daytime load means most generation exports at 0p and you miss the benefit.
• You're in the North – Scotland, Northern England, and Wales get 15–25% less generation due to latitude and cloud cover.
• You have east or west aspect – 15–25% less annual generation vs. south-facing.
• You have heavy shading – chimneys, trees, or nearby buildings casting shadows in winter cut generation by 10–40%.
• You're in a flat with poor aspect – north-facing or heavily shaded flats may not be economic unless you can place panels on a balcony.

How to Know Your Real Numbers: Use the Calculator

These are ballpark figures. Your actual savings depend on:

• Your exact postcode (1–2% variation for every few km south/north)
• Your exact tariff (can swing savings by ±20%)
• Your actual daytime load (self-consumption varies by household)
• Shading (even one shadow reduces generation 10–40%)

To get personalised figures, run the calculator with your postcode, system size, and tariff. It'll give you a far more accurate picture than these averages.

ROI and Payback Context

A 3.4-year payback (averaged across the UK) means:

• Years 1–3.4: Your system is "paying for itself."
• Years 3.4–20: Profit. Pure annual savings with no investment left to recoup.
• 20-year lifetime savings: ~£4,200–£5,100 for an 800W system (assuming no inflation, no tariff rises—both unlikely).

Compared to:

• Savings account – inflation-protected, but pays 4–5% gross interest (~£40 on a £1,000 deposit).
• ISA – tax-free, but ~3–4% real return.
• Premium bonds – technically 0% return (but fun).

Solar's 3.4-year payback, followed by 16+ years of profit, comfortably beats most savings products for a 20-year horizon.

The 15-Year vs 25-Year Assumption

Most calculations assume either a 15-year or 25-year system lifespan. Here's what happens:

15-year scenario (conservative):

• 800W system in London: £256/year × 15 = £3,840 total saving
• Minus initial cost (£800): net £3,040 profit

25-year scenario (realistic for panels):

• 800W system in London: £256/year × 25 = £6,400 total saving
• Minus initial cost (£800): net £5,600 profit

Panels typically last 30+ years with minimal degradation (~0.5%/year). Most failures are inverter or wiring, not panels. Using 25 years is reasonable; 15 years is conservative and under-estimates true return.

Next Steps

1. Know your postcode – it's the biggest lever on your savings.
2. Check your tariff – time-of-use tariffs offer 20–50% better value. Read the tariff guide.
3. Use the calculator – enter your details for a personalised saving estimate.
4. Check for shading – run the Solar Report to see if your property has sun exposure all year.
5. Compare system sizes – 400W, 600W, 800W all have similar payback; pick based on budget and available space.

Key Takeaways

• Typical payback: 3–4 years for a well-placed 800W system in the UK.
• Annual savings: £200–£300 for 800W, depending on location (south pays more than north).
• System size doesn't change payback – only absolute savings. 800W saves twice as much as 400W over the same timeframe.
• Tariff matters hugely – time-of-use rates can add 20–50% to your savings.
• Summer generation is 5× winter – expect seasonal variation in monthly savings.
• 25-year lifespan – panels last far longer than the 3–4 year payback, so most of the saving is pure profit.

Ready to see your exact numbers? Run the calculator with your postcode.